"to begin with" doesn't work there. I agree with Tarheel that "at all" is the better of those choices. His suggested version is even more natural but note that you will hear native speakers put "even" in various positions in that sentence.
I don't even know if there is a god.
I don't know if there even is a god.
I don't know if there is even a god.
Wait, should I replace whether with if? Is whether wrong?
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